Core–Shell Metal–Ceramic Microstructures: Mechanism of Hydrothermal Formation and Properties as Catalyst Materials

Unique metal–ceramic composites with core–shell microarchitecture (γ-Al 2 O 3 @Al and spinel-MeAl 2 O 4 @Al, Me = Zn, Ni, Co, Mn, and Mg) were obtained by a simple hydrothermal surface oxidation (HTSO) of Al metal particles in an aqueous solution of heterometal ions at elevated temperatures (393–473 K). The reactions afforded self-constructed core–shell microarchitecture with Al core encapsulated by the high-surface-area γ-Al 2 O 3 or spinel metal aluminates (MeAl 2 O 4 ) shell with various surface morphologies, compositions, and excellent physicochemical properties. Extensive experimental and theoretical investigation with period 3–6 metal elements (Na +, Ca 2+, Sr 2+, Ba 2+, K +, Fe 3+, Cu 2+, Zn 2+, Ni 2+, Co 2+, Mn 2+, and Mg 2+ ) at various metal concentrations and temperatures revealed that the heterogeneous self-construction of spinel-MeAl 2 O 4 @Al primarily depends on two intrinsic properties of the additive metal ions: (i) thermodynamic stability constant of the metal hydroxide complex and (ii) size of the metal ion. The spinel-MeAl 2 O 4 @Al microstructures formed with a limited number of hetero metal ions (Me = Zn 2+, Ni 2+, Co 2+, Mn 2+, and Mg 2+ ) with (i) moderate rates of the hydroxide formation with compatible kinetics to the hydrolysis of aluminum on the Al surface and (ii) small size of additive metal ions enough for diffusion through the shell layer. As heterogeneous catalyst substrates, these metal–ceramic composites delivered markedly enhanced catalytic performance at intensive reaction conditions because of their facile heat transfer and superior physicochemical surface properties. The performance and effects of the core–shell metal–ceramic composites were demonstrated using Rh catalysts supported on MgAl 2 O 4 @Al. The Rh/MgAl 2 O 4 @Al catalyst was utilized for the endothermic glycerol stream reforming (C 3 H 8 O 3 + 3H 2 O ⇄ 3CO 2 + 7H 2, Δ H 0 298 = 128 kJ mol –1 ), exhibiting markedly greater catalytic performance than the conventional Rh/MgAl 2 O 4 under intensive reaction conditions attributed to significantly facilitated heat transport through the core–shell metal–ceramic microstructures.